Defects in a memory die can occur during manufacturing (e.g., due to contamination) and cause one or more bit lines in the memory to be defective. If the number of defective bit lines is too great, the memory die may need to be discarded. However, if the number of defective bit lines still allows the memory die to meet certain performance criteria, the defective bit lines can be dealt with by replacing the column (i.e., a grouping of multiple bit lines) that contains the defective bit line with a redundant column. Address decoding circuitry can then be configured to use the redundant column any time a request is received to use a bit line in the replaced column.
Bad bit lines caused by defects in the manufacturing process are typically identified during the testing phase of the memory die. However, shortages and open circuits can occur over time as the memory is used in the field, and these shortages and open circuits can cause additional bad bit lines to develop. Bit lines that become bad after manufacturing are sometimes referred to as “grown bad bit lines” and can cause an error in one or more cells of a code word read from the memory. An error correction code (ECC) decoder can be used to detect and correct such errors. However, if the number of errors is too great, the error will be uncorrectable. Accordingly, grown bad bit lines can lead to uncorrectable errors that degrade the reliability of the memory.